An aqueous coating composition comprising an aqueous medium and a water soluble or dilutable resin (sometimes called aqueous resin) and containing in the form of dispersion a hydrophobic resin has been watched with keen interest in paint industries because of the characteristics of being free from air pollution, fire hazard and the like and minimizing the amount of expensive and harmful organic solvent required. Furthermore, in this type of composition, it is able to use a number of combinations of various resins which can develop the characteristics of the respective resins used.
However, in the heretofore proposed aqueous coating compositions containing in the form of dispersion a hydrophobic resin, dispersion particles are in general of relatively large size and when applied, the formed coatings are rather poor in gloss and it is hardly possible to attain the desired level of specular gloss (60.degree.) of 90 or more. Furthermore, since there is a marked tendency of making agglomerates among the dispersed resin particles themselves or in the case of a colored coating composition, among the resin particles themselves as well as between the resin particle and the coexisting pigment particle, causing sediments which are hardly redispersible in the compositions, there are problems awaiting solution in the stability of the coating compositions and in the application characteristics thereof. For these reasons, such compositions have scarcely been used as a top coat for automobile bodies and household electric appliances, requiring a high gloss and surface smoothness.
To cope with the same, a thermosetting aqueous coating composition obtained by the reaction of a composition comprising
(A) a water insoluble acrylic resin having an acid value of 0 to 15 and a hydroxyl value of 30 to 300,
(B) a water soluble or dispersible acrylic resin having an acid value of 15 to 30 and a hydroxyl value of 30 to 300, and
(C) an aminoplast resin and/or pheno-formaldehdye resin, the solid weight ratio of A/B being 20/80 to 80/20 and the solid weight ratio of A+B/C being 60/40 to 95/5, with an amount of basic compound which is sufficient enough to neutralize 80 to 100 mol% of the total carboxyl groups of the composition had been proposed in Japanese Patent Application Kokai No. 164145/82. It was reported that a highly glossy coating with specular gloss (60.degree.) of 90 or more was obtained and excellent stability of coating composition and improved application characteristics were realized.
However, in that invention, both of the water soluble resin and water insoluble resin are limited to particular type acrylic resins. In fact, the inventors of said patent application said that for the attainment of stability of composition and coating appearance, the acid value of the water soluble resin had to be limited to a lower value of 15 to 30, which was far below the level being believed to be appropriate to water soluble resins for coating use. Thus, in the invention of Japanese Patent Application Kokai No. 164145/82, particular acrylic resins are used as water soluble resin and water insoluble resin, respectively, and moreover, solubility of the water soluble resin is limited for the obtainment of improved stability of the coating composition and coating gloss. Nevertheless the actual effects are not fully satisfactory and further improvements are still needed.
As a separate approach, a method was proposed for preparing a resinous dispersion for coating use in Japanese Patent Publication No. 49230/85, wherein water soluble resin, water insoluble resin and water were stirred well in a mixer having a crushing function and then thus obtained emulsion was further treated in a bell type non-driven mixer. In that publication, a statement is given to the effect that though the water soluble resin and the water insoluble resin may be each 100% solid content, a small quantity of hydrophilic organic solvent may be contained in the respective resin for the facility of smooth operation. However, the present inventors have found that when a hydrophilic organic solvent is present in the said composition, there is an undesirable lowering in dispersion stability, causing an inferior storage stability and application characteristics of the composition resulting in a coating which is insufficient in gloss and surface smoothness.
It is, therefore, an object of the invention to provide an aqueous coating composition comprising a water soluble or dilutable resin selected from a wider range of polymers, a water insoluble resin selected from a wider range of polymers, and water, which composition is excellent in storage stability and application characteristics and results in a coating with excellent gloss and surface smoothness. An additional object of the invention is to provide an aqueous coating composition with the aforesaid characteristics, which is formulatable in a simple and easy way and is specifically useful as a top coat for automobile bodies, household electric appliances and the like.
The other objects of the invention shall be clear from the description of the specification and accompanying claims.